1. Field of the Invention
The present invention relates to a system and method for checking a reference plane of a signal trace in a printed circuit board (PCB).
2. Description of Related Art
Printed Circuit Boards (PCBs) are electrical systems, having electrical properties just as complicated as the discrete components and devices mounted to them. PCBs are abstracted to a set of basic physical/electrical structures, including: Traces, Planes, vias, and pads. A trace is a physical strip of metal (usually copper) making an electrical connection between two or more points on an X-Y coordinate of a PCB. Traces carry signals between these points, these points being the 2 or more points on an X-Y coordinate of a PCB. A plane is an uninterrupted area of metal covering an entire PCB layer. Planes may distribute power to a number of points on a PCB or it may be a reference ground connected to a number of grounded points on the PCB. Planes are very important in the process of transmission of signals along traces. A via is a physical piece of metal making an electrical connection between two or more points in the z-space of a PCB. Vias carry signals or power between layers of a PCB.
The combination of a signal trace and a reference (power or ground) plane forms a transmission line. All Input/Output (I/O) signals in a PCB system travel through transmission lines. The Input/Output, or I/O, is the collection of interfaces which different functional units (sub-system) of an information processing system use to communicate with each other or with interfacing signals (information theory). A group of transmission lines makes up a bus. Busses are routed on the PCB such that the signal traces are physically adjacent to one another. Good signal integrity in a PCB system is dependent on having transmission lines with controlled impedance. Impedance is determined by the geometry of the traces and the dielectric constant of a material, this material being in the space around the signal trace and between the signal trace and the reference plane.
Return current is an often neglected aspect of transmission lines and their signal integrity. It is incorrect to assume that a signal trace, by itself, forms a transmission line. Currents flowing in a signal trace have an equal and opposite complimentary current flowing in the reference plane beneath them. The relationship of a trace voltage and a trace current to a reference plane voltage and a reference plane current defines the characteristic impedance of a transmission line formed by a trace and a reference plane. The trace current is the current flowing in the signal trace. The trace voltage is the voltage existed in the signal trace. The reference plane current is the current flowing in the reference plane. The reference plane voltage is the voltage existed in the reference plane. An interruption of reference plane continuity beneath a trace is not as dramatic in effect as severing the signal trace, however, the performance of the transmission line and any devices sharing the reference plane is still affected. Thus, it is important to pay attention to reference plane continuity and return paths, it is also important to ensure that every signal has a closely spaced and fully intact return path.
In general, common wiring software can be controlled and the signal structure can be checked, but the integrity of the reference planes can not be checked. What is needed, therefore, is a system and method for avoiding the return current path discontinuities, where the return current path discontinuities are responsible for many signal integrity problems and the return current path discontinuities are related to the electromagnetic interference (EMI) problems as well.